A so-called linear Fresnel solar heat collecting apparatus is known as a conventional linear type solar heat collecting apparatus formed by arranging solar-heat reflection lines in parallel.
An example of such a conventional linear type solar heat collecting apparatus is shown as Conventional Example 1 in FIG. 11a and FIG. 11b (see Non-patent Document 1 and Patent Document 1). In the linear Fresnel solar heat collecting apparatus of Convention Example 1, a plurality of reflection lines L1, L2, . . . are arranged in parallel on the ground, and reception lines C1, C2, . . . are arranged in parallel with the direction of the reflection lines at predetermined intervals above the reflection lines L1, L2, . . . in such a way as to extend like bridges across the reflection lines. In each reflection line L1, L2, . . . , many rectangular mirror plates H, H, . . . that serve as a heliostat are arranged, and, in each reception line C1, C2, . . . , receivers (solar heat collectors) R, R, . . . are arranged in parallel at a predetermined interval therebetween.
In FIG. 11b, the mirror plates H, H, . . . of each line have their turning angles that are individually controlled so as to turn on a central rotational shaft of each reflection line L1, L2, . . . , and reflect sunlight that has impinged on their specular surfaces, so that light reflected therefrom is received by a nearby receiver R. Heat of the reflected light received by the receiver R is converted into high-temperature water vapor via a heat medium. Each reflection line L1, L2, . . . and the receiver R are disposed in parallel in the south-north direction, and the angle of each reflection line undergoes a rotational adjustment in the east-west direction so that the mirror plates H, H, . . . disposed on each reflection line L1, L2, . . . track the movement of the sun and, as a result, light reflected therefrom is always collected near the receiver R.
The solar heat collecting apparatus is used in, for example, a solar thermal power generation system. A parabolic trough type solar thermal power generation system and a central tower type solar thermal power generation system have already been operated as commercial plants for the solar heat collecting apparatus used in the solar thermal power generation system. In the parabolic trough type solar thermal power generation system, a gutter-shaped parabolic mirror is used, and a heat medium (for example, oil) flowing through the pipe is heated while focusing sunlight on a pipe disposed at the focal position of the mirror, so that electric power is generated by the resulting thermal energy. In the central tower type solar thermal power generation system, a plane mirror provided with a sun tracker is used, and sunlight is collected while focusing sunlight on a solar heat collector of a tower disposed at its central part, so that electric power is generated by the resulting heat collected by a heat medium allowed to flow to the upper part of the tower.
The parabolic trough type solar thermal power generation system is comparatively low in cost, but is low in the temperature of a heated heat medium, and therefore, disadvantageously, has difficulty in achieving high efficiency, whereas the central tower type solar thermal power generation system is capable of obtaining a high-temperature heat medium, but needs a highly accurate light-collecting capability, and therefore, disadvantageously, is high in cost.
On the other hand, the linear Fresnel solar heat collecting apparatus is one of the solar heat collecting apparatuses to which the most attention is paid as a commercial plant for the solar thermal power generation system from the fact that the linear Fresnel solar heat collecting apparatus is more insusceptible to the wind, and has a less rigid, simpler structure, and is higher in land-use efficiency, and therefore can realize a lower power generation cost than the parabolic trough type or the central tower type solar heat collecting apparatus.
Although the linear Fresnel solar heat collecting apparatus is characterized by having a simpler structure and realizing a lower power generation cost than the parabolic trough type or the central tower type power generator as mentioned above, a problem resides in the fact that the optical loss of sunlight becomes great for the following causes, and it is difficult to obtain high light/heat-collecting efficiency.
The optical loss of sunlight is caused by, for example, the fact that a beam of incident light is kicked on a mirror plate (which is called “cosine loss”), the fact that a beam of light reflected by a mirror plate is intercepted by another mirror plate (which is called “blocking”), or the fact that a beam of light incident on a mirror plate is intercepted by another mirror plate (which is called “shadowing”).
Among the causes of the optical loss, the occurrence of “cosine loss” and that of “blocking” are shown in FIG. 12 as an image. “Shadowing” is conspicuously caused especially when incident sunlight diagonally impinges on a mirror, and therefore “shadowing” is not shown in FIG. 12.
Such an optical loss is liable to become great when the inclination angle of a mirror H with respect to the ground level is great or when a change in the rotation angle of a mirror H is great in the angle adjustment of the mirror H performed in accordance with a sun altitude. In FIG. 12, for example, when a receiver R of the conventional linear Fresnel solar heat collecting apparatus is arranged in the south-north direction, rays of light reflected from mirror plates H, H, . . . are apparently radiated toward the single receiver R by inclining the mirror plates H, H, . . . , which have been arranged on the reflection line, in the east-west direction. Therefore, the inclination angle of a mirror with respect to the ground level becomes greater in proportion to an increase in distance of the mirror placed on the reflection line from the receiver R, and the optical loss caused by “cosine loss” and “blocking” is great.
Moreover, in order to follow the tracks of the sun from morning to evening, the mirror plate undergoes a great change in its adjustment angle greater than about Plus or minus an angle of 45 degrees in the east-west direction, and therefore the optical loss caused by “cosine loss” and “blocking” becomes great especially when the sun altitude is low in the morning and evening, and the amount of change of light-collection/heat-collection energy during a day is large. Therefore, the upper limit of temperature obtained as a result of a low degree of light convergence falls within 400 degrees to 500 degrees in water vapor, and a high temperature of 600 degrees or more cannot be obtained.
Additionally, even if many mirror plates are arranged on reflection lines in the east-west direction and even if a wide mirror installation space is provided, the above-mentioned optical loss becomes increasingly greater in mirror plates distant from a receiver, and therefore high light collecting efficiency cannot be obtained. Still additionally, a single receiver has its limitations on the number of mirror plates that radiate reflected light so as to be received by this receiver, and therefore, if many mirror plates are arranged in a wide range, there is a need to assign one reception line to each group of mirror plates the number of which is predetermined, and a receiver R is required to be provided for each reception line, and the heat of reflected light received by each receiver R must be gathered and taken out. For these reasons, the limit temperature obtained by the conventional linear Fresnel solar heat collecting apparatus has been about 500 degrees at the most.
As a method for reducing the above-mentioned optical loss, Patent Document 2 proposes a linear Fresnel solar heat collecting system in which both the longitudinal direction of each reflection line and the longitudinal direction of a receiver are arranged in parallel in the east-west direction. This system is described as Conventional Example 2. Briefly speaking, in the system of Conventional Example 2, each mirror plate H, H, . . . on each reflection line L1 (L2, L3, . . . ) is rotated only in the south-north direction without being rotated in the east-west direction with respect to the track of the sun so as to guide reflected light to the receiver, as shown in FIG. 13.
According to this method, the rotation angle of a mirror in the south-north direction is a small angle not exceeding several degrees during a day (morning and evening), and is a small angle of about Plus or minus an angle of 15 degrees during a year, and therefore the above-mentioned optical loss can be greatly reduced. Therefore, the total amount of heat-collection energy received by the receiver R can also be increased by arranging many mirror plates in the south-north direction and by increasing an installation space for the mirror plates.
However, in the system of Conventional Example 2, the angle of the mirror in the east-west direction cannot be adjusted, and therefore, especially when the sun altitude is low in the morning and evening, a linear light-collection range into which light is converged by the mirror plates greatly deviates from the receiver, and, disadvantageously, the heat-collection energy during a day greatly varies.
Additionally, in either of Conventional Examples 1 and 2, the conventional linear Fresnel solar heat collecting apparatus has a structure in which the reflection line and the receiver are arranged in parallel with each other, and therefore rays of light collected on the receiver become a linear irradiation range that is substantially uniform. Disadvantageously, in this irradiation range, the re-radiation of heat becomes great when the temperature rises to a high temperature of about 600 degrees, and the efficiency of the amount of heat absorbed by a heat medium is easily deteriorated.